Short distance electromagnetic communication for instruments in electrically conductive housings

ABSTRACT

An electromagnetic communication system for a wellbore drilling instrument includes a first antenna disposed in a pressure tight compartment in a wall of a drill collar or a pressure tight sonde and a second antenna disposed in a pressure tight sonde disposed in an interior passage in the drill collar or in a pressure tight compartment in a wall of the drill collar. A wall thickness of the drill collar and/or a wall thickness of the sonde, an electrically conductive material used for the drill collar and the sonde and a frequency of electromagnetic signals applied to at least one of the antennas are chosen to enable electromagnetic communication between the first antenna and the second antenna disposed.

CROSS REFERENCE TO RELATED APPLICATIONS

Priority is claimed from U.S. Provisional Application No. 62/528,291filed on Jul. 3, 2017, which application is incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable.

BACKGROUND

This disclosure relates to the field of instruments used to makemeasurements of drilling parameters and/or formation parameters inwellbores drilled through subsurface formations. More specifically, thedisclosure relates to communication devices for use in such instrumentswhere housings for the instruments and related components are made fromhigh strength, electrically conductive materials.

Measuring parameters related to wellbore drilling and/or properties offormations penetrated by drilling may be made during drilling usingcertain measuring instruments. Such measuring instruments are known inthe art to be disposed in one or more “drill collars”, which arethick-walled tubular housings having connections for inclusion into a“drill string.” Such instruments may further include one or more sensorslocated externally to the drill collar. Such one or more sensors maycomprise a data storage device or memory, and in some cases a shortdistance communication transceiver to communicate measurements and otherdata between the external sensor(s) and the measuring instrumentdisposed in a drill collar. One example of such a sensor andcommunication device is described in U.S. Pat. No. 5,813,480 issued toZaleski, Jr. et al.

Communication devices such as the one described in the '480 patent useelectromagnetic signals to transfer information between the externalsensor(s) and the drill collar based measuring instrument(s). Antennasused in such communication devices may be disposed in a recess formed inthe exterior surface of the drill collar and the instrument (e.g., adrill bit in the case of the device shown in the '480 patent). Therecess may be covered on its exterior by an electrically non-conductivecover, e.g., made from glass fiber reinforced resin, or by a metal covercomprising openings therethrough to enable passage of electromagneticenergy through the cover. Irrespective of the type of cover used, theantennas in such communication devices are exposed, e.g., by beingembedded in insulating material such as elastomer, to fluid in thewellbore. The fluid in the wellbore, which, depending on the verticaldepth of the wellbore, the density of liquid (“drilling mud”) fillingthe wellbore and the pressure required to pump drilling mud through thewellbore and back to the surface, may exert substantial fluid pressure.Such pressure is known to breach pressure barriers, e.g., high pressurefeed through bulkhead connectors, used to connect such antennas toelectronic circuits disposed inside the drill collar and inside ahousing or body for the external sensor(s).

SUMMARY

An electromagnetic communication system for a wellbore instrumentaccording to one aspect includes at least a first antenna disposed ineither a pressure tight compartment in a wall of a drill collar or in apressure tight sonde disposed in an interior passage in the drillcollar. At least a second antenna is disposed in a pressure tight sondedisposed in either an interior passage within the drill collar or withina pressure tight compartment in the wall of the drill collar. A wallthickness of the drill collar and/or a wall thickness of the pressuretight sonde, an electrically conductive material used for the drillcollar and/or the pressure tight sonde and a frequency ofelectromagnetic signals applied to at least one of the antennas arechosen to enable electromagnetic communication between the first antennaand the second antenna.

In some embodiments, the first antenna and the second antenna comprisecoaxially wound coils.

In some embodiments, the antenna in the wall of the drill collarcomprises a solenoid coil disposed on one side of the drill collar.

In some embodiments, a longitudinal axis of the antenna in the wall ofthe drill collar is oriented so that a magnetic dipole moment isparallel to a longitudinal axis of the drill collar.

In some embodiments, a longitudinal axis of the antenna in the wall ofthe drill collar is oriented so that a magnetic dipole moment is at anoblique angle to a longitudinal axis of the drill collar.

In some embodiments, at least one of the first antenna and the secondantenna comprises a main coil and a longitudinal end coil disposed ateach longitudinal end of the main coil.

In some embodiments, one of the first antenna and the second antennacomprises a magnetometer.

In some embodiments, the antenna in the wall of the drill collarcomprises a saddle coil.

In some embodiments, the pressure tight sonde comprises a metal pressurebarrel and an antenna cover disposed at one longitudinal end of themetal pressure barrel.

A method for electromagnetic communication according to another aspectincludes conducting an electromagnetic signal to a transmitter antennaand detecting an electromagnetic signal induced in a receiver antenna bythe electromagnetic signal conducted to the first transceiver antenna.At least one of the transmitter antenna and the receiver antenna isdisposed in a pressure tight compartment in a wall of a drill collar orin a pressure tight sonde, and at least one of the receiver antenna andthe transmitter antenna is disposed in a pressure tight sonde disposedin an interior passage in the drill collar or in a pressure tightcompartment in a wall of the drill collar. A wall thickness of the drillcollar and/or a wall thickness of the sonde, an electrically conductivematerial used for the drill collar and/or the sonde and a frequency ofthe electromagnetic signal conducted to the transmitter antenna arechosen to enable electromagnetic communication between the transmitterantenna and the receiver antenna.

In some embodiments, the antenna in the wall of the drill collar and/orthe antenna in the sonde comprise coaxially wound coils.

In some embodiments, the antenna in the wall of the drill collarcomprises a solenoid coil disposed on one side of the drill collar.

In some embodiments, a longitudinal axis of the antenna in the wall ofthe drill collar is oriented so that a magnetic dipole moment isparallel to a longitudinal axis of the drill collar.

The method of claim 12 wherein a longitudinal axis of the antenna in thewall of the drill collar is oriented so that a magnetic dipole moment isat an oblique angle to a longitudinal axis of the drill collar.

In some embodiments, at least one of the antenna in the wall of thedrill collar and the antenna in the sonde comprises a main coil and alongitudinal end coil disposed at each longitudinal end of the maincoil.

In some embodiments, the receiver antenna comprises a magnetometer.

In some embodiments, the antenna in the wall of the drill collarcomprises a saddle coil.

In some embodiments, the sonde comprises a metal pressure barrel and anantenna cover disposed at one longitudinal end of the metal pressurebarrel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an example of communication between devicesmounted in a drill collar and devices mounted in one or more “sondes”,i.e., pressure resistant housings, disposed in an internal passageinside a drill collar.

FIGS. 1A through 1D show different embodiments of a communicationdevice.

FIG. 2 shows an example embodiment of transceiver antennas formed aswire coils wound about a longitudinal axis of a sonde and a drillcollar.

FIG. 3 shows another example embodiment wherein the transceiver antennain the drill collar is formed as a solenoid coil wound about an axisparallel to the longitudinal axis of the drill collar.

FIG. 4 shows another embodiment corresponding to the embodiment of FIG.3 wherein the transceiver coil in the drill collar is oriented along anoblique axis.

FIG. 5 shows an example embodiment wherein the transceiver antenna inthe sonde is substituted by a magnetometer.

FIG. 6 shows another example embodiment using a magnetometer in thesonde and wherein an antenna in the drill collar is formed as a saddlecoil.

FIGS. 7A, 7B and 7C show response of a transceiver antenna pair forvarious relative locations between locations of the antennas withrespect to the receiver.

FIG. 7D shows a graph of receiver response indicative of the fact thatfor certain longitudinal separation distances between the transceiverantenna in the drill collar and the transceiver antenna in the sonde, a“dead zone” exists.

FIG. 8 shows another example embodiment of transceiver antenna that cansubstantially eliminate the dead zone shown in FIG. 7D.

FIGS. 8A through 8C show responses of various longitudinal positions ofthe antennas with respect to each other.

FIG. 8D shows a graph similar to FIG. 7D wherein the dead zone of FIG.7D is substantially eliminated.

FIG. 9 shows graphs of skin depth for various metals with respect toelectromagnetic signal frequency.

DETAILED DESCRIPTION

FIG. 1 shows schematically the principle of electromagneticcommunication between components of a wellbore measuring instrumentsystem according to the present disclosure. One or more communicationnodes 14, 16 may be disposed within the wall of a drill collar 10. Spacefor the communication nodes 14, 16 in the wall of the drill collar 10may be formed, for example and without limitation by machining a spacethrough the exterior of the wall of the drill collar 10 and covering thespace with a pressure-tight cover 11A made from high strength materialto form a pressure tight compartment 11 in the wall of the drill collar10. For example, the drill collar 10 may be made from high strength,non-magnetic material such as monel, certain alloys of stainless steelor alloys sold under the trademark INCONEL, which is a registeredtrademark of Huntington Alloys Corp., Huntington, W. Va. If used, such acover may be made from a similar material. In some embodiments, thecompartment 11 may be made by machining from within an interior passage10A within the interior of the drill collar 10.

One or more “sondes”, shown in FIG. 1 at 12A and 12B may comprise atubular housing that is pressure sealed at each longitudinal end andformed from a similar material, a different material or the samematerial as the drill collar 10. The sonde 12 and its sealedlongitudinal ends may define a pressure-tight interior space suitablefor additional communication nodes, e.g., at 18 and 20. The sonde 12A,12B may be disposed in the interior passage 10A (show as usuallycentered in the drill collar 10 but not so limited). In the presentcontext, the term “node” is intended to mean an electromagnetictransmitter antenna, an electromagnetic receiver antenna or atransceiver antenna, and associated operating circuitry, shown generallyat 18A. The operating circuitry 18A may be associated with or connectedto one or more sensors 19 disposed in, at, on or proximate any or all ofthe nodes 12A, 12B, 18 and 20. Signal communication between any or allof the nodes may enable communication of measurements made by any of thesensors 19 to a part of a sonde or a part of a drill collar wherein maybe disposed a device (not shown) for storing measurements made by thevarious sensors (e.g., solid state memory) and/or for communicating someof such measurements to the surface using known wellbore to surfacecommunication devices, e.g. and without limitation, drilling mud flowmodulation.

FIG. 1A shows two antennas 22, 22A each disposed in a respective sonde12, 12A. The antennas 22, 22A may be coils wound about a common axis 13,and such antennas 22, 22A and may be separated by a longitudinaldistance D such that an electromagnetic field emitted by one of the twoantennas 22, 22A is within the sensitive region of the other antenna22A, 22 to enable electromagnetic signal communication between theantennas 22, 22A. In the embodiment shown in FIG. 1A, either antenna 22,22A may perform either or both the functions of transmitter andreceiver, the illustration being provided only to show the longitudinaldistance between the antennas 22, 22A to effect the requiredelectromagnetic communication between them.

FIG. 1B shows another example embodiment in which two sondes 12, 12A aredisposed proximate each other inside a collar 10 (or more than onecollar, e.g., adjacent collars in a drilling tool string). In theexample embodiment of FIG. 1B, one sonde 12A comprises a coil 22 whichmay be used as a transmitter antenna. The other sonde 12 may comprise amagnetometer 28 which may be used as an electromagnetic receiver.

FIG. 1C shows another embodiment of a sonde 12, wherein the sonde 12comprises a pressure tight housing, enclosure or barrel 12C which may bemade from electrically conductive metal as in the previously describedembodiments. Electronic circuitry 15 used to provide signals to anantenna coil 22 and/or receive signals from the antenna coil 22 may bedisposed inside the pressure tight barrel 12C. The antenna coil 22 maybe disposed longitudinally beyond an end of the pressure tight barrel12C inside an antenna cover 22B. The antenna cover 22B may be partiallyconically shaped to reduce erosion wear on the antenna cover 22B whenfluid, e.g., drilling fluid, is pumped through the collar (10 in FIG.1B). The antenna cover 22B may be made from electrically conductivemetal for pressure resistance, but may also be made from materials suchas plastic, ceramic and various resins such as polymer resin or epoxyresin. It will be appreciated that the shape and thickness and type ofthe materials used for the antenna cover 22B may be selected to providepressure resistance so that fluid under pressure is excluded fromentering the pressure tight barrel 12C. The embodiment in FIG. 1C may beused for sonde to sonde communication or sonde to collar communication,the latter being possible if a communication node such as shown at 14 inFIG. 1 is disposed in the wall of the collar.

FIG. 1D shows another embodiment of a communication system that has asonde 12 with an antenna coil 22 as described with reference to FIGS. 1Aand 1B disposed inside a first collar 10A. A crossover sonde 12D may bedisposed in a second collar 10B threadedly connected to the first collar10A. The crossover sonde 12D may provide a pressure tight enclosure thatis sealingly connected to a corresponding pressure sealed chamber orcompartment 10B1 in the second collar 10B. The embodiment shown in FIG.1D may be used to communicate between a sonde and a collar across athreaded connection between adjacent collars, e.g., first and secondcollars 10A, 10B so as to reduce wear caused by erosion at theconnection interface, where erosion could be caused by flow of fluid,e.g., drilling fluid, through the first and second collars 10A, 10B.

FIG. 2 shows an example embodiment of a transceiver antenna 22 disposedin a compartment 11 in the drill collar 10. The transceiver antenna 22may comprise a coil wound about the longitudinal axis 11 of the drillcollar 10 wherein the coil windings are substantially in planesperpendicular to the longitudinal axis 13 and are substantially coaxialwith the longitudinal axis 13. Field lines for the magnetic component ofan electromagnetic field are shown at 23. The electromagnetic field fromthe transceiver antenna 22 (in FIG. 2 operating as an electromagnetictransmitter) pass through a transceiver antenna 24 disposed in a sonde12 located in the drill collar 10. FIG. 2 illustrates the principle thatthe transceiver antenna 22 disposed in the collar 10 and the transceiverantenna 24 disposed in the sonde 12 may be longitudinally displaced fromeach other by a selected distance D and still provide signalcommunication between the antennas 22, 24.

FIG. 3 shows another example embodiment similar in principle to theembodiment shown in FIG. 2, but wherein the transceiver antenna 26 inthe compartment 11 in the drill collar 10 is formed as a solenoid withcoils wound in planes perpendicular to the longitudinal axis (13 in FIG.2). The magnetic component field lines are shown at 23 in FIG. 3 and maybe characterized as generally being symmetric about the transceiverantenna 26, but generally disposed in the compartment 11 on one side ofthe drill collar 10, in contrast to the field lines in FIG. 2 which aresymmetric about the longitudinal axis (13 in FIG. 2) of the collar 10.

FIG. 4 shows another example embodiment similar to the embodiment ofFIG. 3, wherein the transceiver antenna 26 in the drill collar 10 isdisposed at an oblique angle with respect to the longitudinal axis (11in FIG. 2). A possible advantage of the embodiment shown in FIG. 4 isthat the transceiver antenna 26 in the compartment 11 in the drillcollar 10 and the transceiver antenna 24 in the sonde 12 may beseparated from each other by a larger longitudinal distance than thatdescribed with reference to the embodiments shown in FIG. 2 and FIG. 3while maintaining electromagnetic signal communication between theantennas 24, 26.

FIG. 5 shows another example embodiment wherein the transceiver antennain the sonde 12 may be substituted by a magnetometer 28. A transmitterantenna 26 may be disposed in the compartment 11 in the drill collar 10.The transmitter antenna 26 may be configured similarly to thetransceiver antenna shown in and explained with reference to FIG. 3. Itwill be appreciated by those skilled in the art that electromagneticcommunication using an embodiment such as shown in FIG. 5 can only takeplace in one direction, i.e., from the transmitter antenna 26 to themagnetometer 28. In some embodiments, the positions of the transmitterantenna and the magnetometer may be reversed in order to enablecommunication from the sonde 12 to the drill collar 10. In someembodiments, a magnetometer may be disposed in both the drill collar 10and the sonde 12, and a corresponding transmitter antenna may bedisposed in the drill collar 10 to enable two-way communication. Themagnetic dipole orientation of the transmitter antenna 26 and thesensitive axis of the magnetometer 28 may be along any selecteddirection provided that the magnetic field component of the signalproduced by the transmitter antenna 26 is at least in part along thesensitive direction of the magnetometer 28.

In another embodiment shown in FIG. 6, the transmitter antenna may be inthe form of a “saddle coil” 30, which may be configured as a wire loophaving sides 30A that substantially conform to the curvature of thedrill collar 10 and longitudinal segments 30B having length selectedsuch that the saddle coil 30 encloses a selected area. The sonde 12 maycomprise a magnetometer 28 as shown in FIG. 6, or may comprise anotherantenna of types shown in and explained with reference to FIGS. 2 and 3.

FIGS. 7A, 7B and 7C illustrate graphically that the longitudinalseparation between a first transceiver antenna 32 and a secondtransceiver antenna 34 may affect the degree of signal coupling betweenthem. In the example shown in FIGS. 7A, 7B and 7C, the antennas 32, 34may be in the form of solenoid coils such as shown in and explained withreference to FIG. 2. When the antennas 32, 34 are at a same longitudinalposition with respect to each other, as shown in FIG. 7A, the magneticcomponent field lines 33 are such that there is substantial signalcoupling between the antennas 32, 34. A graph of response of theantennas 32, 34 is shown at curve 40 in FIG. 7D at position A. When theantennas 32, 34 are separated by a particular longitudinal distance, asshown in FIG. 7B, the magnetic component field lines from one antenna 32are perpendicular to the longitudinal magnetic dipole moment of theother antenna 34. At such distance, and as shown at curve 40 in FIG. 7Dat Position B, there is substantially no signal coupling between theantennas 32, 34. Such distance may be referred to as a “dead zone.”Further separation of the antennas 32, 34 as shown in FIG. 7C may resultin restored signal coupling between antennas 32, 34 as shown by curve 40in FIG. 7D at Position C.

FIG. 8 shows one example embodiment of a transceiver antenna arrangementthat can substantially eliminate the dead zone. The first transceiverantenna is shown at 32A and in this embodiment may comprise a main coil1 and two longitudinal end coils 2A, 2B. The magnetic component fieldlines from the main coil 1 are shown at 33 in FIGS. 8A, 8B and 8C. Themagnetic component field lines are shown for each of the longitudinalend coils 2A, 2B at 33A and 33B, respectively, in each of FIGS. 8A, 8Band 8C. When the two antennas 32A, 34 are longitudinally coincident asshown in FIG. 8A and correspondingly at Position A in FIG. 8D, the maincoil 1 is in primary electromagnetic communication with the antenna 34.When the longitudinal position of the antennas 32A, 34 is as shown inFIG. 8B and at Position B in FIG. 8D, which in the embodiment shown inFIG. 7C is in the “dead zone”, the magnetic component field lines fromone of the longitudinal end coils 2B provides signal communicationbetween such longitudinal end coil 2B and the antenna 34. As thelongitudinal spacing between the antennas 32A, 34 increases, signalcommunication between the main coil 1 and the antenna 34 may increase inamplitude as shown in the graph at 40. In the graph at 40 it may beobserved that the dead zone shown in FIG. 7D has been substantiallyeliminated. Thus, a configuration of antennas as shown in FIG. 8 mayenable a larger range of longitudinal separation between the antennas32A, 34 than when using only a single solenoid coil for both antennas(as shown in FIG. 7).

The longitudinal end coils 2A, 2B may be driven at a different frequencyand/or at different times relative to the main coil 1. For example, if achosen center frequency for a selected embodiment is Fo, then the maincoil 1 may be driven at Fo+Fd, where Fd represents a frequencydifference related to the selectivity of receiver circuitry connected tothe antenna 34. In such embodiment, the longitudinal end coils 2A, 2Bmay be driven at Fo−Fd. The frequency Fo+/−Fd may be chosen to provideadequate signal coupling between the coils 1, 2A, 2B and the antenna 34(with some minor attenuation). Adequate coupling is explained below withreference to FIG. 9. Some embodiments may use the same frequency Fo orfrequency range Fo+/−Fd as above, but switch transmitter current betweenthe main coil 1 and the longitudinal end coils 2A, 2B to avoidsubstantial attenuation and/or corruption due to waveform super-positionof simultaneously transmitted signals through all three coils 1, 2A, 2B.

Various embodiments as shown in and explained with reference to FIGS. 1through 8 may provide communication between antennas that do not requirethe antennas to be at the same longitudinal position with respect toeach other. The foregoing property of embodiments made according to thepresent disclosure may reduce the need for design of electromagneticcommunication components which have precise longitudinal and axialalignment, thus enabling more tolerance in assembly, in ability to recutthreads on collars and in design of centralizing devices that retainsondes inside collars.

FIG. 9 shows a graph of skin depth with respect to frequency for variouselectrically conductive materials that may be used for the drill collar(10 in FIG. 2) and the sonde (12 in FIG. 2). The material selected maybe that which provides sufficient skin depth at a selected frequencysuch that electromagnetic communication may take place between antennasas explained with reference to the various embodiments in FIGS. 1through 8 while the wall thickness of the material is selected toprovide sufficient mechanical strength to withstand both the mechanicalstresses resulting from drilling (e.g., for the drill collar) as well asthe fluid pressure expected at the deepest point in a wellbore. Anon-limiting example of wall thickness is that required to withstandfluid pressure of 20,000 pounds per square inch. In such embodiments,the material selected and the frequency selected should be those whichprovide an adequate ratio of skin depth to wall thickness such thatelectromagnetic signal is detectable between any two antennas, inparticular but not limiting, within a drill collar wall and within asonde. Example materials that may be used in some embodiments includestainless steel alloy 316 and INCONEL brand alloy 718.

Although only a few examples have been described in detail above, thoseskilled in the art will readily appreciate that many modifications arepossible in the examples. Accordingly, all such modifications areintended to be included within the scope of this disclosure as definedin the following claims.

What is claimed is:
 1. An electromagnetic communication system for awellbore instrument, comprising: at least a first antenna disposed in apressure tight compartment in a wall of a drill collar or in a pressuretight sonde disposed in an interior passage in the drill collar; and atleast a second antenna disposed in a pressure tight sonde disposed in aninterior passage in the drill collar or in a pressure tight compartmentin the wall of the drill collar; wherein a wall thickness of the drillcollar and/or a wall thickness of the pressure tight sonde, anelectrically conductive material used for the drill collar and thepressure tight sonde and a frequency of electromagnetic signals appliedto at least one of the antennas are chosen to enable electromagneticcommunication between the first antenna and the second antenna.
 2. Thesystem of claim 1 wherein the first antenna and the second antennacomprise coaxially wound coils.
 3. The system of claim 1 wherein theantenna in the wall of the drill collar comprises a solenoid coildisposed on one side of the drill collar.
 4. The system of claim 3wherein a longitudinal axis of the antenna in the wall of the drillcollar is oriented so that a magnetic dipole moment is parallel to alongitudinal axis of the drill collar.
 5. The system of claim 3 whereina longitudinal axis of the antenna in the wall of the drill collar isoriented so that a magnetic dipole moment is at an oblique angle to alongitudinal axis of the drill collar.
 6. The system of claim 3 whereinat least one of the first antenna and the second antenna comprises amain coil and a longitudinal end coil disposed at each longitudinal endof the main coil.
 7. The system of claim 1 wherein one of the firstantenna and the second antenna comprises a magnetometer.
 8. The systemof claim 1 wherein the antenna in the wall of the drill collar comprisesa saddle coil.
 9. The system of claim 1 wherein the sonde comprises ametal pressure barrel and an antenna cover disposed at one longitudinalend of the metal pressure barrel.
 10. A method for electromagneticcommunication, comprising: conducting an electromagnetic signal to atransmitter antenna; detecting an electromagnetic signal induced in areceiver antenna by the electromagnetic signal conducted to the firsttransceiver antenna; wherein at least one of the transmitter antenna andthe receiver antenna is disposed in a pressure tight compartment in awall of a drill collar or in a pressure tight sonde; and at least one ofthe receiver antenna and the transmitter antenna is disposed in apressure tight sonde disposed in an interior passage in the drill collaror in a pressure tight compartment in a wall of the drill collar;wherein a wall thickness of the drill collar and/or a wall thickness ofthe sonde, an electrically conductive material used for the drill collarand the sonde and a frequency of the electromagnetic signal conducted tothe transmitter antenna are chosen to enable electromagneticcommunication between the transmitter antenna and the receiver antenna.11. The method of claim 10 wherein the antenna in the wall of the drillcollar and/or the antenna in the sonde comprise coaxially wound coils.12. The method of claim 10 wherein the antenna in the wall of the drillcollar comprises a solenoid coil disposed on one side of the drillcollar.
 13. The method of claim 12 wherein a longitudinal axis of theantenna in the wall of the drill collar is oriented so that a magneticdipole moment is parallel to a longitudinal axis of the drill collar.14. The method of claim 12 wherein a longitudinal axis of the antenna inthe wall of the drill collar is oriented so that a magnetic dipolemoment is at an oblique angle to a longitudinal axis of the drillcollar.
 15. The method of claim 12 wherein at least one of the antennain the wall of the drill collar and the antenna in the sonde comprises amain coil and a longitudinal end coil disposed at each longitudinal endof the main coil.
 16. The method of claim 10 wherein the receiverantenna comprises a magnetometer.
 17. The method of claim 10 wherein theantenna in the wall of the drill collar comprises a saddle coil.
 18. Themethod of claim 10 wherein the sonde comprises a metal pressure barreland an antenna cover disposed at one longitudinal end of the metalpressure barrel.